A fairly general definition of
architecture in the system space (versus civil or building architectures), is:

Architecture
is the high-level definition of the structure of a system, which is comprised of
parts, their interrelationships, and externally visible properties.

With this definition in mind, it is
all the more obvious that Enterprise Architecture is more than the collection of
the constituent architectures (Business, Application, Technology, and
Information). The interrelationships among these architectures, and their joint
properties, are essential to the Enterprise Architecture. That is to say, these
architectures should not be approached in isolation. Together, they are intended
to address important Enterprise-wide concerns, such as:

meeting stakeholder needs

aligning IT with the
business

seamless integration and data
sharing

security and dependability

data integrity, consistency

reducing duplication

Treating the Enterprise as a system,
means taking the interactions among the constituent architectures into account.
By the same token, the whole point of breaking a system into parts is so that
task is less overwhelmingly complex, and specialists can focus on the parts and
make progress.